In recent years, aliphatic polyester has received attention as a resin that produces a small environmental load. Originally, the aliphatic polyester has been employed mainly for the purpose of exploiting its characteristic, biodegradability. However, in fact, the characteristic of biodegradability has effectively been utilized solely in applications such as agriculture and fishery where the aliphatic polyester is used outdoors and is difficult to recover. On the other hand, applications such as cushioning packaging materials or heat insulating materials, which are likely to be used in environments of high temperature and high humidity or require long-term resistance, had the problem of being unavailable if they were highly degradable.
Recently, a characteristic valued highly instead of biodegradability for environmentally friendly resins is that biomass materials derived from plants are used as the raw materials thereof. In such situations, polylactic acid that has been produced in large amounts at low cost by the polymerization of lactic acid derived from plants started to rank as the dominant aliphatic polyester.
As described above, polylactic acid has an excellent characteristic of producing a small environmental load and being relatively inexpensive. However, the polylactic acid presents the following problems when used as a raw material for a foam:
The first problem is that polylactic acid has a small melt tension and results in a closed cell content decreased due to the burst of air bubbles generated in a foaming process. When air bubbles burst, foaming agents within the air bubbles dissipate outside the air bubbles. As a result, the air bubbles stop growing, and therefore, an expansion ratio is decreased. A foam having a low expansion ratio, that is, a high density, does not sufficiently acquire characteristics such as light weights, cushioning properties, and heat insulating properties. For example, only a thin foam sheet having a density on the order of 0.3 g/cm3 is obtained in Patent Document 1.
The second problem is that polylactic acid softens over a glass transition temperature of usually 50 to 60° C. due to its low allowable temperature limit and becomes deformed by a load applied thereto. Polylactic acid composed of highly pure L-lactic acid or D-lactic acid as a raw material is crystalline. Its heat resistance is enhanced with increases in the degree of crystallinity. However, the crystallization speed of polylactic acid is generally exceedingly slow. High-temperature curing for a long time is required for increasing the degree of crystallinity. Furthermore, polylactic acid loses crystallinity and becomes amorphous when the purity of L-lactic acid or D-lactic acid is lowered and the content of the lactic acid unit contained in a lower amount exceeds approximately 10%. Patent Document 2 has disclosed that the use of an amorphous polylactic acid having a particular melt viscosity can enhance foamability. However, this polylactic acid is amorphous and therefore has a low heat resistance.
Third, polylactic acid lacks flexibility when used in applications such as cushioning materials where a foam is compressed. Therefore, there arises a problem that the extent to which the foam that has been compressed and crushed recovers a thickness is small.
To solve these problems, a method is used wherein a polyolefin resin or polystyrene resin having high foamability and high heat resistance is blended with polylactic acid. However, the problem in this case is that highly polar polylactic acid is incompatible with the nonpolar polyolefin resin or polystyrene resin and therefore results in a nonuniform mixture that does not foam, or if the mixture foams, a closed cell content is decreased due to exfoliation occurring in the interface between them. In Patent Document 3, highly polar organic acid ester having an epoxy group capable of reacting with the terminal group of polylactic acid is used as a compatibilizer component for blending polylactic acid and a polyolefin resin. However, only a foam in a thin sheet form having a low expansion ratio and a small section is obtained.
[Patent Document 1] JP-A-04-304244
[Patent Document 2] JP-A-2002-317066
[Patent Document 3] WO 2006/103969